The present invention relates to self-assembled metal colloid monolayers, methods of preparation, and use thereof.
In surface enhanced Raman scattering (SERS), million-fold enhancements in Raman scattering can be obtained for molecules adsorbed at suitably rough surfaces of Au, Ag, and Cu. Although many approaches have been reported, preparation of well-defined, stable SERS substrates having uniform roughness on the critical 3 to 100 nm scale has proven difficult. Because colloidal Au can be synthesized as monodisperse solutions throughout most of this size regime, and because molecules adsorbed to closely spaced colloidal Au and Ag exhibit enhanced Raman scattering, these particles are excellent building blocks for SERS-active substrates. The key issue is whether colloidal Au and Ag particles can be organized into macroscopic surfaces that have a well-defined and uniform nanometer-scale architecture. Indeed, controlling nanostructure is currently a central focus throughout materials research. Progress in self assembly of organic thin films on metal surfaces [C. D. Bain and G. M. Whitesides, Angew. Chem. Int. Ed. Engl. 28, 506 (1989); A. Ulman, An Introduction to Ultrathin Organic Films, from Langmuir-Blodgett to Self-Assembly (Academic Press, Boston, 1991)] led us to explore the reverse process: self assembly of colloidal Au and Ag particles onto supported organic films. As detailed below, this approach has yielded surfaces that are SERS-active, characterizable at both the macroscopic and microscopic levels, highly reproducible, electrochemically addressable, and simple to prepare in large numbers. Moreover, these substrates have a surface roughness that is defined by the colloid diameter (which is tunable) and an average interparticle spacing that is continuously variable. As such, self-assembled Au, Ag and Ag-coated colloid monolayers are likely to have extraordinary utility for SERS.
In the nearly twenty years since the discovery of surface enhanced Raman scattering (SERS) of molecules adsorbed at roughened Ag electrodes, and the accompanying theoretical work demonstrating the need for surface roughness, there have been numerous reports of new architectures for SERS substrates. See, for instance, Liao, P. F.; Bergman, J. G.; Chemla, D. S.; Wokaun, A.; Melngailis, J.; Hawyrluk, A. M.; Economou, N. P. Chem. Phys. Lett. 1981, 82, 355-9; Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790-8; Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 120, 435-55; Tran, C. D. Anal. Chem. 1984, 56, 824-6; Soper, S. A.; Ratzhlaff, K. L.; Kuwana, T. Anal. Chem. 1990, 62, 1438-44; Sequaris, J.-M.; Koglin, E. Fresenius J. Anal. Chem. 1985, 321, 758-9; Aroca, R.; Jennings, C.; Kovacs, G. J.; Loutfy, R. O.; Vincett, P. S. J. Phys. Chem. 1985, 89, 4051-4; Moody, R. L.; Vo-Dinh, T.; Fletcher, W. H. Appl. Spectrosc. 1987, 41, 966-70; Ni, F.; Cotton, T. M. Anal. Chem. 1986, 58, 3159-63; Yogev, D.; Efrima, S. J. Phys. Chem. 1988, 92, 5761-5; Goudonnet, J. P.; Bijeon, J. L.; Warmack, R. J.; Ferrell, T. L. Phys. Rev. B: Condensed Matter 1991, 43, 4605-12; Murray, C. A.; Allara, D. L. J. Chem. Phys. 1982, 76, 1290-1303; Brandt, E. S. Appl. Spectrosc. 1993, 47, 85-93; Alsmeyer, Y. W.; McCreery, R. L. Anal. Chem. 1991, 63, 1289-95; Mullen, K.; Carron, K. Anal. Chem. 1994, 66, 478-83; Beer, K. D.; Tanner, W.; Garrell, R. L. J. Electroanal. Chem. 1989, 258, 313-25; Dawson, P.; Alexander, K. B.; Thompson, J. R.; Haas III, J. W.; Ferrell, T. L. Phys. Rev. B: Condens. Matter 1991, 44, 6372-81; Roark, S. E.; Rowlen, K. L. Appl. Spectrosc. 1992, 46, 1759-61; Roark, Shane E.; Rowlen, K. L. Chem. Phys. Lett. 1993, 212, 50; Roark, Shane E.; Rowlen, K. L. Anal. Chem. 1994, 66, 261-70; Walls, D.; Bohn, P. J. Phys. Chem. 1989, 93, 2976-82; Dutta, P. K.; Robins, D. Langmuir 1991, 7, 2004-6; Sheng, R.-S.; Zhu, L.; Morris, M. D. Anal. Chem. 1986, 58, 1116-9.
These surfaces span a wide range of assembly principles and encompass similarly broad levels of complexity. Examples of SERS-active surfaces include electrochemically-roughened electrodes, microlithographically-prepared elliptical Ag posts, aggregates of colloidal Au or Ag particles--both in solution and associated with chromatographic media, evaporated thin films, Ag-coated latex particles, substrates prepared by chemical reduction of Ag.sup.+, and liquid Ag films. The motivation for this work stems from several intrinsically attractive aspects of SERS as a vibrational spectroscopy-based structural tool and/or analytical method: million fold signal enhancements compared to solution Raman spectra, adsorption-induced fluorescence quenching, a lack of interference from H.sub.2 O, and molecular generality. However, while SERS has been invaluable for certain narrowly defined applications, most spectroscopists would agree that the technique has not lived up to its enormous potential.
The problem has been the inability of any previous surface to meet all, or even most, of the essential criteria that would define a truly usefull SERS substrate: strongly enhancing, reproducible, uniformly rough, easy to fabricate, and stable over time. Biocompatibility is also extremely important, insofar as previous studies demonstrating partial or full protein denaturation upon adsorption to SERS-active substrates [Holt, R. E.; Cotton, T. M. J. Am. Chem. Soc. 1989, 111, 2815-21; Lee, N.-S.; Hsieh, Y.-Z.; Morris, M. D.; Schopfer, L. M. J Am. Chem. Soc. 1987, 109, 1353-63] have proven to be a major setback to the use of SERS in biological systems. Other desirable characteristics include electromagnetic tunability (i.e. the ability to control the wavelength where optimal enhancement occurs, so as to match the substrate to the photon source), electrochemical addressability--to control the extent of adsorption and the redox state of adsorbed species, a lack of surface "activation" steps, and a low cost per substrate. This last feature is particularly important for applications involving large numbers of routine measurements, such as in environmental monitoring, airport security, and clinical medicine.